A signal generator provides an electronic signal having a desired level corresponding to a waveform data that is previously set by a user. The electronic signal is generally an analog signal provided to circuits or instruments under test for calibrating and/or testing the operational parameters of the circuit or the instrument.
FIG. 1 is a block diagram of an example of an output amplifier circuit 52 usable in a signal generator. Positive and negative current sources 60 and 62 generate positive and negative currents according to an input voltage Vi. The difference between the currents is provided to a 50 ohm termination (output) resistor R50 that converts the current to an output voltage Vo. Relatively large voltage sources, such as ±32V, are used for the current sources 60 and 62 to provide the positive and negative currents to generate the output voltage Vo having a dynamic range of ±10V at the 50 ohm termination.
High speed operational amplifiers U110 and U210 are used for driving of transistors Q110 and Q210 in response to the input signal. Most of high speed operational amplifiers have a power voltage range around ±5V. The operational amplifiers U110 and U210 use the power voltages ±5V from ±27V, that is, from ±32 V to ±22 V respectively. Because the input voltage Vi is around 0V, the level must be shifted to around the power voltages of the operational amplifiers U110 and U210. Therefore, positive and negative voltage to current converters 64 and 66 are used to produce varying currents depending on the input voltage Vi. The positive and negative voltages to current converters 64 and 66 are coupled to positive and negative bias voltages via resistors R100 and R200. The bias voltages are fixed to achieve the maximum dynamic range of the amplifier circuit.
The positive current source 64 has an operational amplifier U100 having its non-inverting input coupled to a reference ground. Junction J11 is virtually grounded due to the negative feedback of the operational amplifier U100. The current into the inverting input of the operational amplifier U100 is the result of current I101 flowing in a resistor R101 in response to the input voltage Vi and current I100 flowing in a resistor R100 in response to the positive bias voltage. Because of the operational amplifier U100, the currents I100 and I101 also follow in a resistor R102, which produces the voltage of a junction J12. The voltage at the junction J12 and resistance values of resistors R102 and R103 set the emitter current of the transistor Q100 as well as the current flowing in R111 since the collector current of Q 100 is almost the same as the emitter current. The voltage at junction J13 is fixed at +27V as a result of the non-inverting input of operational amplifier U110 being coupled to a +27V source. Current changes in the output of the positive current source 60 are the result of voltage changes at junction J14 caused by changes in the current through resistor R111 in response to the input voltage Vi. The negative current source 62 works in a similar manner with current changes in the output of the negative source 62 being the result of voltage changes at junction J24 caused by changes in the current through resistor R211 in response to the input voltage Vi.
The described amplifier circuit is an A class amplifier and consumes a large amount of power due to the large currents flowing through the transistors Q110 and Q210 even though the output voltage is small. This leads to high heat generation that is not preferable for stable operation of the circuit and accelerates degradation of the components.